1. Field of the Invention
This invention relates to a capacitor microphone unit, and more particularly to a capacitor microphone unit in which an open end of a unit case is crimped in order to fixedly attach and reliably shield components.
2. Description of the Related Art
An existing capacitor microphone unit will be outlined with reference to FIG. 1 (related to the present invention) of the accompanying drawings. A cylindrical case 10 has a bottom, is formed by the drawing compound process, and houses a ring 26, a diaphragm 20, a fixed electrode 22, an insulator 24 and a circuit board 30, all of which are inserted in the named order. A peripheral top edge of the cylindrical case 10 is folded inward, and is crimped. This part is called the “crimp 14”. The crimp 14 firmly holds a peripheral edge of the circuit board 30, so that the components are fixed in the cylindrical case 10. In FIG. 1, the cylindrical case 10 is shown upside down. A plurality of circular openings 12 are formed on the bottom of the cylindrical case 10. An inner peripheral edge of the circuit board 30 holds a peripheral edge of the insulator 24. The insulator 24 holds the fixed electrode 22 via its inner surface. A periphery of the diaphragm 20 is sandwiched between an outer periphery of the fixed electrode 22 and the ring 26. There is a space between the diaphragm 20 and the fixed electrode 22 except for the peripheral edge of the fixed electrode 22. Hence, the diaphragm 20 is vibrated by voices getting into the cylindrical case 10.
The crimp 14 of the cylindrical case 10 appropriately presses the components and makes them immovable. An evaporated surface of the diaphragm 20, the ring 26 and the case 10 are electrically connected to a ground wiring pattern of the circuit board 30. This is effective in blocking noise caused by high frequency signals. The capacitor microphone shown in FIG. 1 is of an electret type, in which electric charges are semi-permanently held on the diaphragm 20. The diaphragm 20 and the fixed electrode 22 are formed as a capacitor. A capacitance of the capacitor varies with vibrations, so that electric charges are discharged or introduced. Hence, an amount of electric charges also changes, so that a minute current is produced in response to vibrations of the diaphragm 20. The minute current is converted into a voltage signal at high resistance. The voltage signal is converted to low impedance by an amplifier, and is discharged outward. The amplifier is included in the circuit board 30, and is realized by a field effect transistor 28 (FET) functioning as an impedance converter.
At present, a number of capacitor microphones are very popular in order to convert voices into electric signals in a cellular phone using high frequency signals. High frequency signals entering into a microphone unit are detected by the impedance converter constituted by the FET 28, are converted into audio frequency signals, and are mixed as noise into an audio signal converted by the microphone unit.
The crimp 14 of the cylindrical case10 is brought into pressure contact with the ground wiring pattern of the circuit board 30 in order to accomplish an electric connection, and protects the microphone unit against noise caused by high frequency signals. This feature is inevitable in cellular phones or the like using high frequency signals, and is very effective in protecting capacitor microphones against noise caused by high frequency signals.
The printed circuit board 30 structured as shown in FIG. 6, which shows a wiring pattern on an outer surface. Most of the wiring pattern is used as a ground wiring pattern 60, which surrounds not only the remaining wiring patterns but also the periphery of the printed circuit board 30. The ground wiring pattern 60 is flat as a whole.
However, when the ground wiring pattern 60 is flat, pressure applied by the crimp 14 is dispersed, and the crimp 14 is in partial contact with the ground wiring pattern 60. In an extreme case, the crimp 14 is only in point contact with the ground wiring pattern 60. Therefore, high frequency signals will not be sufficiently blocked, and may enter into the microphone unit and cause noise. Therefore, it is very difficult to check with eyes whether or not the crimp 14 and the ground wiring pattern 60 are in partial contact with each other since contact is very minute. Further, it is also very difficult to measure a resistance value using a tester because the resistance itself is very minute.
Japanese Patent Laid-Open Publication No. Hei 11-155,197 describes a capacitor microphone unit. In the publication, a shielded case houses a circuit board block at one end thereof, and a support of a capacitor microphone picking up sound pressure as a variation of capacitance is positioned at the other end thereof. The support, capacitor microphone and circuit board block are held in unison in the shielded case. The circuit board block includes a grounding pattern which is conductively connected to the shielded case. An elastic microphone cap is attached to an outer surface of the shielded case. A conductive tape is attached on the outer surface of the shielded case, and is connected to the grounding pattern via a part thereof.
In the foregoing publication, the capacitor microphone unit includes the shielded case in which the capacitor microphone, support and circuit board block are crimped. However, the crimped part of the shielded case and the ground wiring pattern of the circuit board block are not directly pressed. The ground wiring pattern and the outer surface of the shielded case are made conductive via the conductive tape. Electrical conduction is unstable between the ground wiring pattern and the shielded case, so that the capacitor microphone is susceptible to external high frequency signals.
There has been proposed a microphone unit in which a microphone unit case has its open end folded in order to hold an outer periphery of a circuit board (refer to Japanese Patent Laid-Open Publication No. Hei 06-339,192).
The present invention has been contemplated in order to overcome problems of the related art.